Re: [PATCH 4/4 RFC] rcu: New rcu_user_enter_irq() andrcu_user_exit_irq() APIs

[Paul E. McKenney]

On Tue, Nov 29, 2011 at 03:05:23PM +0100, Frederic Weisbecker wrote:
> On Mon, Nov 28, 2011 at 05:00:36PM -0800, Paul E. McKenney wrote:
> > On Mon, Nov 28, 2011 at 01:53:23PM -0800, Josh Triplett wrote:
> > > On Mon, Nov 28, 2011 at 10:24:47PM +0100, Frederic Weisbecker wrote:
> > > > A CPU running in adaptive tickless mode wants to enter into
> > > > RCU extended quiescent state while running in userspace. This
> > > > way we can shut down the tick that is usually needed on each
> > > > CPU for the needs of RCU.
> > > 
> > > Very awesome.  I've wanted to see this change for a long time.  Thanks!
> > 
> > I am a fan, also.  ;-)
> > 
> > [ . . . ]
> > 
> > > > @@ -503,6 +515,18 @@ void rcu_user_exit(void)
> > > >  	__rcu_idle_exit();
> > > >  }
> > > >  
> > > > +void rcu_user_exit_irq(void)
> > > > +{
> > > > +	unsigned long flags;
> > > > +	struct rcu_dynticks *rdtp;
> > > > +
> > > > +	local_irq_save(flags);
> > > > +	rdtp = &__get_cpu_var(rcu_dynticks);
> > > > +	WARN_ON_ONCE(rdtp->dynticks_nesting != 1);
> > > > +	rdtp->dynticks_nesting = (LLONG_MAX / 2) + 1;
> > > > +	local_irq_restore(flags);
> > > > +}
> > > > +
> > > 
> > > Any chance that either of these two needs a memory barrier of some kind,
> > > to prevent leakage of operations from between them?  Or can you count on
> > > no RCU-protected operations occurring during (or leaking into) the
> > > extended quiescent state?
> > 
> > There is no need for a memory barrier on rdtp->dynticks_nesting because
> > it is used (aside from state dumping) only by the local CPU.  In contrast,
> > changes to ->dynticks are visible to other CPUs, hence the memory barriers
> > around changes to ->dynticks.
> > 
> > Information flows within the CPU from ->dynticks_nesting to ->dynticks,
> > which is externally visible.
> > 
> > Frederic, given my hamhandedness on the first patch and given that you
> > mentioned its being less time critical, I will let you forward port
> > patches #3 and #4.  I have pushed the first two patches to -rcu, branch
> > rcu/dyntick.  I will be testing over the evening.
> 
> Sure. Also #3 and #4 are not used upstream, so I should probably rather
> carry these in my tree once I do a rebase against yours.

Works for me!  I am hoping that my dyntick-idle work is near an end.
For this round, anyway.  (Fifth or sixth rework!)

							Thanx, Paul

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